Amplifiers, such as high-power amplifiers used in the base stations of wireless communication systems, typically exhibit non-linearity over their operating ranges. This non-linearity can result in noise that can corrupt or otherwise interfere with the communications. To address this problem, additional circuitry may be added to an amplifier in an attempt to linearize the effective amplifier response. Conventional techniques for linearizing amplifiers typically involve pre-compensation and/or feed-forward compensation.
In amplifier linearization based on pre-compensation, the input signal that is to be amplified is pre-distorted prior to being applied to the amplifier in order to adjust the input signal based on known non-linearities in the amplifier transfer function. In feed-forward compensation, an auxiliary signal is fed forward and combined with the output of the amplifier to adjust the output signal for non-linearities in the amplifier transfer function.
FIG. 1 shows a high-level block diagram of prior art amplifier system 100, which is linearized using pre-compensation. In particular, a sample of an RF input signal x(t) is applied to envelope detector 102, which generates a voltage representative of the instantaneous envelope power level of the input signal. This analog power signal p(t) is digitized by analog-to-digital converter (ADC) 104. The resulting digital power signal p(n) is applied to pre-distorter 106, which generates a pre-distorted signal in the form of digital pre-distortion component signals I(n) and Q(n). These digital pre-distortion signals I(n) and Q(n) are converted to analog pre-distortion signals I(t) and Q(t) by digital-to-analog converters (DACs) 108. The analog pre-distortion signals I(t) and Q(t) are used by vector modulator 110 to modulate a delayed version of the RF input signal x(t) delayed by delay line 114 to generate a pre-distorted RF signal y(t) that is then input to amplifier 112, which generates an amplified RF output signal z(t). The purpose of RF delay line 114 is to compensate for the processing time of elements 102-108 to ensure that vector modulator 110 uses appropriately time-aligned pre-distortion signals I(t) and Q(t) to modulate the RF input signal x(t). (Although FIG. 1 shows pre-distortion being applied to the input signal in the analog domain using a vector modulator, in alternative embodiments, pre-distortion can be applied to a baseband representation of the input signal in the digital domain.)
Delaying RF signals can be expensive (e.g., about $1 for each nanosecond of delay). As such, one of the goals in implementing amplifier system 100 is to reduce the overall processing time of elements 102-108 to keep the delay required to be imparted by RF delay line 114 as small as possible. Moreover, the larger the delay line, the greater the attenuation of the RF signal. As such, gain is typically added to the amplifier to account for this attenuation, resulting in further cost and increased distortion.